1. Field of the Invention
The present invention relates to a non-destructive testing apparatus for a pipe. More particularly, the present invention relates to an apparatus for generating and measuring bending vibration in a non-ferromagnetic pipe without physical contact, which may generate and measure bending vibration in a pipe buried underground or a pipe covered by insulating materials.
2. Description of the Related Art
Pipes are frequently buried underground or covered by insulating materials, coating materials, etc. In the related art, it is possible to measure bending vibration and to perform safety inspection in the pipes only after the pipes are uncovered by digging the ground or by peeling the insulating materials, the coating materials, etc.
However, there is a drawback in the related art that it needs long time and great cost for both a test and recovery after the test. Thus, it requires development of an apparatus for detecting structural defects of pipes without digging the ground or peeling the insulating materials, the coating materials, etc.
Magnetostrictive effect, which is also called Joule effect, refers to a phenomenon that mechanical deformation occurs in a ferromagnetic material when the ferromagnetic material is placed in magnetic fields. A reciprocal phenomenon of the magnetostrictive effect (i.e. a phenomenon that changes in magnetic field in vicinity of materials occur when mechanical stress is applied to the materials) is designated as inverse magnetostrictive effect or Villari effect.
The magnetostrictive effect can be expressed as follows.
When ferromagnetic materials are placed in magnetic field H, induced magnetic flux density B inside the ferromagnetic materials is the sum of magnetic flux density in vacuum and magnetic flux density induced by magnetization of the ferromagnetic materials. Therefore, the magnetic flux density B inside the ferromagnetic materials can be expressed as equation 1.B=μ0H+μ0M=μ0H+μ0χmH=μ0(1+χm)H=μ0μrH=μH   [Equation 1]
where B denotes magnetic flux density, H denotes magnetic field intensity applied by an outside magnet or electromagnet, M denotes magnetization, χm denotes magnetic susceptibility, μ0 denotes permeability of free space, μr denotes relative permeability, and μ denotes permeability.
The Joule effect and Villari effect can be expressed as equations 2 and 3.
                    ɛ        =                              σ                          E              H                                +                                    q              *                        ⁢            H                                              [                  Equation          ⁢                                          ⁢          2                ]            B=μσH+qσ  [Equation 3]
where ε denotes strain, σ denotes stress applied to the ferromagnetic material, EH denotes modulus of elasticity in a constant magnetic field and μσ denotes permeability when constant stress is applied.
The coefficients q* and q in equations 2 and 3 representing the Joule effect and Villari effect can be represented as equations 4 and 5,respectively.
                                                        q              *                        =                                          ⅆ                ɛ                                            ⅆ                H                                              )                σ                            [                  Equation          ⁢                                          ⁢          4                ]                                                      q            =                                          ⅆ                B                                            ⅆ                σ                                              )                H                            [                  Equation          ⁢                                          ⁢          5                ]            
If an apparatus for generating and measuring bending vibration use magnetostrictive effect or inverse magnetostrictive effect mentioned above, the apparatus for generating and measuring bending vibration can measure deformation of testing member without any physical contacts with the testing member by applying magnetic field to the testing member and by measuring variation of the magnetic field according to the inverse magnetostrictive effect. Thus, the apparatus for generating and measuring bending vibration can be applied to various testing members inaccessible or difficult to access.